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Petrographical and Geophysical Investigation of the Ecca Group

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Petrographical and Geophysical Investigation of the Ecca Group Open Geosci. 2019; 11:313–326 Research Article Christopher Baiyegunhi*, Zusakhe Nxantsiya, Kinshasa Pharoe, Temitope L. Baiyegunhi, and Seyi Mepaiyeda Petrographical and geophysical investigation of the Ecca Group between Fort Beaufort and Grahamstown, in the Eastern Cape Province, South Africa https://doi.org/10.1515/geo-2019-0025 depth slices result, dolerite intrusions are pervasive in the Received Mar 20, 2018; accepted Mar 13, 2019 northern part of the study area, extending to a depth of about 6000 m below the ground surface. The appearance Abstract: The outcrop of the Ecca Group in the Eastern of dolerite intrusions at the targeted depth (3000 - 5000 m) Cape Province was investigated in order to reveal petro- for gas exploration could pose a serious threat to fracking graphic and geophysical characteristics of the formations of the Karoo for shale gas. that make up the group which are vital information when considering fracking of the Karoo for shale gas. The pet- Keywords: Density, dolerites, Ecca Group, gravity, mag- rographic study reveals that the rocks of the Ecca Group netic, porosity are both argillaceous and arenaceous rock with quartz, feldspar, micas and lithics as the framework minerals. The sandstones are graywackes, immature and poorly sorted, thus giving an indication that the source area is near. The 1 Introduction observed heavy minerals as well as the lithic grains signify This study focuses on road cut exposures of the Ecca Group that the minerals are of granitic, volcanic and metamor- along road R67 between Fort Beaufort and Grahamstown phic origin. The porosity result shows that of all the forma- (Figure 1). The Ecca Group is the second largest subdivi- tions that make up the Ecca Group, the Whitehill Forma- sion of the Karoo Supergroup in Southern Africa; it covers tion is the most porous with an average porosity of about more than 50% of the South Africa’s land surface and it is 2.1% and also least dense with an average dry density of believed to have been emplaced during the Early Permian. about 2.5 g/cm3. The least porous unit is the Ripon For- The Karoo is a semi-desert region of Southern Africa with mation with porosity of about 0.8% but has the highest vast distribution of sedimentary rocks that covers an area dry density of approximately 2.8 g/cm3. The magnetic map of up to 700 000 km2 [1]. The Karoo Supergroup owes its shows some ring-like structures which coincide with do- name to the Karoo region in which it was accumulated in lerites that were mapped in the field. As revealed by the an environment believed to be a retro-arc foreland basin environment [2]. To date, there is controversy surround- ing the origin and development of the Karoo Basin. The *Corresponding Author: Christopher Baiyegunhi: Department basin is well known worldwide because of its fossil con- of Geology and Mining, University of Limpopo, Private Bag X1106, tent that record an unbroken stratigraphic succession that Sovenga 0727,South Africa; Department of Geology, Faculty of span from the Carboniferous to the Middle Jurassic. Re- Science and Agriculture, University of Fort Hare, Private Bag X1314, cently, it was envisaged that the Karoo basin developed Alice, 5700, Eastern Cape Province, South Africa; Email: [email protected] due to blocks subsidence along the marginal faults [3, 4]. Zusakhe Nxantsiya, Kinshasa Pharoe: Department of Geology, The Ecca Group is a sedimentary geological unit that Faculty of Science and Agriculture, University of Fort Hare, Private comprises mostly of shales and sandstones. Almost all of Bag X1314, Alice, 5700, Eastern Cape Province, South Africa; Council South Africa’s coal resources and one-third of the coal re- for Geoscience, 280 Pretoria Street, Silverton, Pretoria 0184, South sources in the southern hemisphere are in the rocks of the Africa Ecca Group [5]. The coal seams is described to be horizon- Temitope L. Baiyegunhi, Seyi Mepaiyeda: Department of Geology, Faculty of Science and Agriculture, University of Fort Hare, Private tal throughout in the Main Karoo Basin except where the Bag X1314, Alice, 5700, Eastern Cape Province, South Africa seams are associated with dolerite intrusions [1]. The dis- Open Access. © 2019 C. Baiyegunhi et al., published by De Gruyter. This work is licensed under the Creative Commons Attribution 4.0 License 314 Ë C. Baiyegunhi et al. Figure 1: Geological map of the study area (Modified from [1]). The inserted small red colour box shows the study area. Table 1: Lithostratigraphy of the Ecca Group in the Eastern Cape Province of South Africa [1]. Group Formation Lithology Special Characteristics Ages Ecca Waterford Sandstone, mudstone Water escapes structures, slumping, ball and 258 - 252 Ma pillow structures Fort Brown Sandstone shale, Rhythmites 262 - 258 Ma Ripon Sandstone, shale Wave ripples, Glossopteris, rhythmites 267 - 262 Ma Collingham Shale, claystones, tuff, Thinly bedded, yellow tuff, Glossopteris, trace 274 - 270 Ma siltstones, sandstones fossils Whitehill Black carbonaceous Weathered white shale , pyritic, gypsum at 275 Ma shale, chert surface, dolomite concretions Prince Albert Khaki shale, siltstones Pyrite, quartz veins, pyritic, sharks teeth, 280 - 275 Ma foraminifera Petrographical and geophysical investigation of the Ecca Group between Fort Beaufort and Grahamstown Ë 315 tribution, lateral extent, thickness and maceral content roo Supergroup [13]. The term Ecca was suggested by Ru- of these coal seams are controlled by basin tectonic and bidge (1858) in [13] for argillaceous sedimentary strata ex- differential subsidence [1, 6]. The Ecca group is also be- posed in the Ecca Pass, near Grahamstown in the Eastern lieved to be hosting the much debated shale gas with more Cape Province, South Africa. Thus, the use of the term Ecca emphasises on the Whitehill and Collingham Formations outside the Main Karoo Basin is sometimes questionable in the southern Karoo. Since the discovery of natural gas as the rock types could be totally different. The group com- deposit in the carbonaceous shale of the Whitehill and prises of shale, mudstones, siltstones, sandstones, minor Collingham Formations, the Ecca Group has become fa- conglomerate and coal. mous; however, local exploration started in the 1960s but Generally, the Karoo Supergroup is stratigraphically was later stopped in the late 1970s due to poor technical subdivided into five main groups, namely; the Dwyka know-how [7, 8]. (Late Carboniferous), Ecca (Early Permian), Beaufort (Late Shale gas has become important in the last few years Permian–Middle Triassic), Stormberg (Late Triassic–Early and the main exploration target is the Ecca Group due to Jurassic) and Drakensberg Groups (Middle Jurassic) [5]. the recent developments in technology on how to recover The Drakensberg lavas are believed to have terminated sed- the natural gas from the host shale. Fracking or fractur- imentation in the basin in the Middle Jurassic [14]. The ing of the Karoo to extract shale gas has attracted more rocks of the Ecca Group were first identified by [15] as interest from researchers and economists. This is due to the “Ecca Beds” and were separated from the overlying the credence that shale gas is an economically feasible fos- “Beaufort Beds” (now known as the Beaufort Group). The sil fuel for generating cheap and clean electricity. Thus re- “Ecca Beds” were at a later stage renamed the “Ecca Se- ducing environmental pollution as well as help in meeting ries” by [16] in his fourfold sub-division of the “Karoo Sys- the increasing demands of electricity in South Africa. This tem”. [17] subdivided the Ecca “Series” in the southern study was undertaken in order to provide more informa- Karoo Basin into a Lower, Middle and Upper Ecca Stage. tion on the petrographic and geophysical characteristics The Ecca Series in the southern Karoo Basin was later re- of the Ecca Group in the Eastern Cape Province of South named by [18] as the Ecca Group. At the same time, the Africa. Lower, Middle and Upper Ecca "Stages" became formal- ized as the Ripon, Fort Brown and Waterford Formations. The underlying Prince Albert and Whitehill Formations 1.1 Geological background were also included into the Ecca Group as the formations that make up the Lower Ecca Group, although they had The Main Karoo Basin of South Africa hosts the thickest been part of the upper Dwyka. [18] later renamed the grey and stratigraphically most complete mega- sequence of and yellow shales on top of the Whitehill Formation as the several depositories of the Permo-Carboniferous to Juras- Collingham Formation. With slight alterations this strati- sic age sediments in southwestern Gondwana continent [2]. graphic outline was received by the South African Com- Several models have been proposed for the formation of mittee for Stratigraphy [19] and to date this stratigraphic the basin. The interpretation of the evolution and tec- outline for the southern Karoo Basin is used. The stratigra- tonic setting of the Karoo Basin vary from a transient phy of the Ecca Group is however complex and this com- hypothetical mantle plume related model [9], a retro-arc plexity is related to its mode of origin, deep marine wa- foreland basin formed as a result of shallow angle sub- ter and shallow marine turbidites and submarine fan de- duction of the palaeo-Pacific plate underneath the Gond- posits from specific source areas [13]. The marine clays wana supercontinent [1], a transtensional foreland system and mudstones of the Prince Albert Formation were de- formed as a result of subsidence and tilting in a strike- posited on the diamictites of the Dwyka Group in the south- slip regime [3, 4], extensional back-arc basin formed due eastern part of the Karoo Basin [20]. This was followed by to oblique subduction of the palaeo-Pacific plate under the carbonaceous shale of the Whitehill Formation.
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